The LTE protocol currently being developed and implemented provides a high-speed wireless communication network for use by devices such as mobile phones and data terminals. The LTE protocol itself is detailed in various documents, for example as published by the 3rd Generation Partnership Project (3GPP).
HARQ processes proposed for LTE transmit blocks of data along with error detection and forward error correction (FEC) bits. In incremental redundancy HARQ, if the errors in a received data block cannot be corrected, the transmitter is informed via Negative Acknowledgement (NACK) and the data block is retransmitted. The retransmission is coded differently from the previous transmissions and retransmissions of the same data block; that is in accordance with a different redundancy version (RV). The receiver may then combine different redundancy versions to improve the probability of successfully decoding the data block. This type of HARQ effectively lowers the coding rate with each retransmission. HARQ effectively improves coverage, however due to signalling and processing delays, only one redundancy version can be transmitted every 8 ms.
TTI bundling has been proposed as a modification to the HARQ mechanism in order to provide improved coverage without dramatically increasing latency and signalling overhead. A detailed overview of TTI bundling can be found in “HARQ Operation in case of UL Power Limitation,” Ericsson, June 2007, 3GPP TDoc R2-072630. Essentially, rather than waiting for a NACK before transmitting the next redundancy version of a data block, several redundancy versions are transmitted in consecutive transmit time intervals (TTIs). The HARQ feedback is sent after the last redundancy version of the data block is received. Current implementations specify transmission of four redundancy versions at a time. To preserve backward compatibility, further retransmission (if triggered due to a NACK) is delayed until the 16th TTI after the first redundancy version was sent.
Increasing the TTI bundle size beyond 4 RVs is a possible way to improve coverage. For example, a 3rd Generation Partnership Project (3GPP) work item entitled “Updated SID on: Provision of low-cost MTC UEs based on LTE” (TSG RAN meeting #57, Chicago, USA, September, 2012, RP-121441), relates to a new study requiring a +20 dB improvement in coverage for LTE systems. To obtain such a +20 dB improvement through TTI bundling would require a TTI bundle size of 400 RVs.
The current implementation of TTI bundling and HARQ mechanisms of LTE is heavily influenced by the need to transmit voice conversations, for example via Voice over IP (VoIP). For these implementations, a fixed TTI bundle size is logical since there is a very limited delay tolerance and typically no time to perform more than one HARQ retransmission.
Therefore there is a need for a new data transmission mechanism.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present technology. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present technology.